Field of the Disclosure
The disclosure relates to a method of remotely measuring strain response of a test material by optical methods using a thin multi-layer assembly.
Description of the Prior Art
U.S. Patent Publication No. 2012/0176629 A1, entitled “Remote Displacement Sensor, Including an Optical Strain Gauge, an Assembly and System Therewith”, was published on Jul. 12, 2012, based upon PCT/US10/048921. This patent application, by the same inventor as the present application, discloses a remote displacement sensor, such as an optical strain gauge, which uses an optical amplifier implemented by patterns, such as, but not limited to, moire patterns, to calculate changes in position or gauge length. In the embodiment implemented as a strain gauge with moire patterns, two foil layers are provided, a lower foil layer with a reference or static moire pattern generated by the overlaying of a first pattern with parallel lines at a first fundamental frequency and a second pattern with parallel lines at a second fundamental frequency. The lower foil layer further includes a first section with a first pattern with parallel lines at the first fundamental frequency while the upper layer provides a second section with a second pattern with parallel lines at the second fundamental frequency. The overlaying of the foils causes an overlying of the first and second sections thereby causing a moire pattern of the same wavelength as the reference pattern. However, relative movement of the two foils perpendicular to the parallel lines, in response to a movement in the gauge length in response to strain on the specimen, causes a phase change in the overlaid pattern which is greater than the relative movement. The image of the optical strain gauge is captured by a camera or other optical device and the resulting image is processed by a Fast Fourier Transform or similar algorithm to determine the phase change, thereby calculating the change in gauge length and therefore the resulting strain.
While this application is well-adapted to its intended purposes, further improvements to this disclosure are sought.
Additionally, with respect to other clip-on extensometers, composite materials are very stiff and tend to break explosively in tensile testing. This prevents the use of clip-on type of extensometers because they are typically damaged by the forces of the break. These types of extensometers are expensive, costing thousands of dollars, and are therefore not intended to be single-use devices. Non-contact optical extensometers, often costing more than $50,000 are typically not able to measure strain to the accuracy necessary to determine correct modulus on such stiff materials. Measuring modulus on composite materials typically requires strain accuracy error less than 20 micro-strain units (a gauge length change of 20 parts per million) at very low strain levels, typically in the 0.1%-0.6% strain range. This is equivalent to sub-micron displacement measurement accuracy at a gauge length of 50 millimeters.
The composites industry therefore relies on the standard bonded strain gauge to achieve the necessary strain accuracy and be within acceptable cost as a single use device. The bonded gauge consists of a precisely etched sheet of thin metal foil about 10 millimeter by 10 millimeter (typically using integrated circuit level microlithography accuracy) that is epoxy bonded onto the surface of the surface of the specimen. It measures strain by producing tiny changes of electrical resistance as it is stressed. To measure these subtle resistance changes it is electrically wired into an external bridge amplifier circuit. Before bonding the metal foil the specimen surface has to be specially prepared by machining a precisely flat surface, polishing and then removing any residual debris by use of a chemical bath.
The bonded gauge process is typically as follows for every specimen tested—machine, polish and clean the surface of the specimen; chemical wash (often 2 baths); very carefully position the foil gauge wherein alignment is critical given its short active length; prepare and apply a uniform layer of epoxy over the device and wait for drying; solder wires to the metal pads on the device and connect the specimen to external electrical bridge circuit when it is mounted in load frame
These steps add up to significant installation labor time for every test. Additionally, the costs of bare foil strain gauge ranges can be considerable. Statistically, the preparation steps can be potentially damaging to the specimen material due to, for example, possible effects of the chemicals used and cuts and dings into the specimen.
Further prior art includes art includes U.S. Pat. No. 7,047,819 entitled “Testing of Samples” by Haywood; U.S. Pat. No. 6,075,893 entitled “Computer Controlled Optical System for Angular Alignment of Structures Using Moire Patterns” to Brandstetter; U.S. Pat. No. 6,164,847 entitled “Image Parameter Detection” to Roy Allen (the present inventor); U.S. Pat. No. 2,787,834 entitled “Grating Strain Gauges” to Shoup; DE 3120653 A1 entitled “Device for Determining Movement parameters and Creep States of Materials” to Ludwig and EP 0255300 A2 entitled “High Sensitivity Strain Detector” to Buckingham and Blackwood.